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Thread: Failed Ignition Circuit on Rotax 912SP

  1. #1

    Failed Ignition Circuit on Rotax 912SP

    One of the ignition circuits on my 912 SP Quik has failed - or fails after a short time when the engine has warmed up. This morning, a mate from the Bristol Microlight Aircraft Club spent several hours trying to find the problem with the following results. Both ignition switches work, and so do the two electronic modules (one per circuit) on top of the engine (when the inputs are switched over, the problem transfers to the switch that normally operates the good circuit). The resistances of the 4 'pickups' bolted onto the front end of the engine casing are well within what the 912 Operator's Manual indicate. Furthermore the gap between the pickups and the triggers on the generator appear to be normal. All the wiring seems to be in order. What is puzzling is that for the first few minutes, both circuits work - as they did this morning.

    Has anyone had this problem? Has anyone got any ideas as to what could be the problem and how to fix it?

    John Sparks

    G-MRJJ

  2. #2

    Failed Ignition Circuit on Rotax 912SP

    Hi John

    Are you checking systems after they have warmed up and Failed ? sounds like you have a electrical connection that is making and braking with either heat or vibration the black boxes have been known to brake wires at the point of entry into the moulding i have also seen the pick ups stop working when hot ..will be a dificult one to find ..

    Rgs Mike

  3. #3

    Failed Ignition Circuit on Rotax 912SP

    Hi Mike,

    Systems were checked after one circuit had failed and in today's temp. of -2 degs C the barely warmed engine was cool enough to need the choke every time it was cstarted.

    Yes - I feel quite an expense coming on!!!

    JOHN

  4. #4
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    Failed Ignition Circuit on Rotax 912SP

    John,
    For what it is worth I had several similar problems with the ignition system on my 912s and they all boiled down to the fine leads coming out of the ignition boxes. Unless they are firmly strapped down, they can vibrate and I assume fatigue. They then create irritating intermittent faults. Garry Masters sorted mine out and touch wood they have been OK since. I think Rotax have taken more care over the securing of these leads on later engines. Hopefully you may have the same problem which is fairly inexpensive.
    While on the subject of 912S ignition systems, I have fitted the Conair Softstart module to mine and to date I have found the starting so much more gentle. I have a CT2k in which I have done nearly 700 hours. My engine is fitted with the slipper clutch and I understand from Conair that if you don't have a slipper clutch you do not gain very much by adding the Softstart. Incidentally, I created a minor mod for the CT which is accepted by the BMAA

    Good Luck John

    Peter Walker

  5. #5

    Failed Ignition Circuit on Rotax 912SP

    John,
    As peter says, you will probably find a broken wire, It will most likely be one of the two red wires coming from the stator which feed each ignition system. Carefully pull on the wires and you will find the broken one will stretch. or using a multi meter measure the resistance to earth of each wire. It has been known for the wire to break quite a way down inside the metal shielding.

    Chris

  6. #6

    Failed Ignition Circuit on Rotax 912SP

    Hi Chris and Peter,
    Thanks for the advice. Firstly, last Summer I fitted a soft-start module which certainly made for a smoother firing up but, of course, does nothing for 'the great wobble' on shut-down. Also, I think that the connections to both the ignition boxes are OK since these were tested by swapping over the input cables, and the fault followed the connection coming from one set of trigger units on the engine. Mike Batchelor - who did the checking - reckons that it it can only be - well here is what he has written:

    [3]'Electrical Resistance checks of all four trigger coils revealed no apparent faults with any coil. All recorded a resistance of around 240 Ohms with no earth faults. [/3]

    [3][/3]

    [3]The trigger coils gaps were checked (with some difficulty!). All appeared to be correct at around .014-.015. The Overhaul manual limits are quoted as .012 - .016. [/3]

    [3][/3]

    [3]While no apparent defect was found with any of the trigger coils it was concluded that the defect must be in one of the green/black coded coils. The red/blue coils supplying the No 1 upper system functioned correctly.'[/3]

    [3][/3]

    [3]Ity is still a puzzle why both circuits are functioning when the engine is started.[/3]

    [3]The Rotax 2-strokes on my XL, Q, and Quaser never missed a beat. Now I have a reliable 4 stroke with just over 300 hrs on the clock and it has failed...... That's progress. Happy New Year folks.[/3]

    [3]JOHN[/3]

    [3][/3]

    [3] [/3]

  7. #7

    Failed Ignition Circuit on Rotax 912SP

    john, If it is completely dead on one circuit then I doubt it would be any of the pick ups (it uses two per ignition circuit and would probably result in one pair of cylinders dead on the faulty mag side or missfiring )The only thing that is checkable on the pick ups is the resistance of each one, the continuity of the wiring and the gap.
    I would still go for one of the red supply wires from the generator. have you tried disconnecting the "soft start" ?
    Of course it could be one of the two electronic box's.
    Chris

  8. #8
    Junior Member
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    Failed Ignition Circuit on Rotax 912SP

    John,

    Just a thought, is it the A or B box that does not work? Con.
    did put in his fitting instructions that if there is a problem try without the soft start. I might be worth having a word with him. I found him to be very helpful.

    Very interesting this. Please keep us up to date.

    Peter

  9. #9

    Failed Ignition Circuit on Rotax 912SP

    Just to keep you up to date - everything has been rejected as the cause of this failure except the wiring from one of the pairs of triggers and, as it is almost unheard of for two to fail at the same time (according to Conrad), we reckon it has to be the plug that leads to the lower electronic module on top of the engine. The up side that goes into the module is OK because that circuit works when the inputs are changed over. So, despite the fact that there is no sign of a breakage in any of the wiring, there has be a fault somewhere around there. The next job is to take that plug apart - when its a bit warmer.
    John

    G-MRJJ

  10. #10
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    Failed Ignition Circuit on Rotax 912SP

    Thank you John,
    We are still listening for the sting in the tail.
    Peter

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